1. Field of the Invention
The present invention relates to a spin valve sensor with a composite pinned layer structure for improving biasing of a free layer structure with reduced sense current shunting and, more particularly, to a composite pinned layer structure which has at least one layer of cobalt iron hafnium oxide (CoFeHfO).
2. Description of the Related Art
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from a moving magnetic medium, such as a rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. A magnetic moment of the free layer is biased parallel to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the rotating disk. An antiferromagnetic pinning layer interfaces the pinned layer for pinning a magnetic moment of the pinned layer 90xc2x0 to the ABS. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from the parallel position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The parallel position is also known as the quiescent position which is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers is minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another in response to a signal field scattering is minimal and when their magnetic moments are antiparallel in response to an opposite signal field scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
The transfer curve (magnetoresistive coefficient dr/R or readback signal of the spin valve head versus applied signal from the magnetic disk) of a spin valve sensor is a substantially linear portion of the aforementioned function of cos xcex8. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative magnetic fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve read head be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is located midway between the maximum positive and negative readback signals and is considered to be zero. With the direction of the magnetic moment of the free layer parallel to the ABS, and the direction of the magnetic moment of the pinned layer perpendicular to the ABS, the bias point is located at zero in a quiescent state of the sensor and the positive and negative readback signals will be equal when sensing positive and negative magnetic fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer, namely a ferromagnetic coupling field HF between the pinned layer and the free layer, a demag field HD from the pinned layer, and sense current fields HI from all conductive layers of the spin valve except the free layer. It is important that these forces position the magnetic moment of the free layer parallel to the ABS so that the bias point is located at a zero position on the transfer curve.
When the sense current IS is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer and the first spacer layer, which are all on one side of the free layer, impose sense current fields on the free layer that rotate the magnetic moment of the free layer toward a first direction perpendicular to the ABS. The pinned layer demagnetization field HD further rotates the magnetic moment of the free layer toward the first direction counteracted by a ferromagnetic coupling field HF of the pinned layer that rotates the magnetic moment of the free layer toward a second direction antiparallel to the first direction.
Since the conductive material on the pinned layer side of the free layer is significantly greater than the conductive material on the other side of the free layer the sense current fields from the pinned layer side are a major force on the free layer which is difficult to counterbalance with the other magnetic forces acting on the free layer. Further, the conduction of the sense current IS through metallic layers of the spin valve sensor, other than the spacer layer, in effect shunts a portion of the sense current which reduces the amplitude of the signal detected by the read head. If less current is shunted through the conductive layers, other than the spacer layer, this can result in more sense current IS being conducted through the spacer layer to increase signal detection.
If the pinned layer is an antiparallel (AP) pinned layer structure instead of a single pinned layer the aforementioned problems are exacerbated. The AP pinned spin valve sensor differs from the simple spin valve sensor in that the AP pinned spin valve sensor has an AP pinned structure that has first and second AP pinned layers instead of a single pinned layer. An AP coupling layer is sandwiched between the first and second AP pinned layers. The first AP pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second AP pinned layer is immediately adjacent to the free layer and is antiferromagnetically coupled to the first AP pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the AP coupling layer between the first and second AP pinned layers. Accordingly, the magnetic moment of the second AP pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first AP pinned layer. The magnetic moments of the first and second AP pinned layers of the AP pinned structure subtractively combine to provide a net magnetic moment that is less than the magnetic moment of the single pinned layer. The direction of the net moment is determined by the thicker of the first and second AP pinned layers. A reduced net magnetic moment equates to a reduced demagnetization (demag) field HD from the AP pinned structure. Since the exchange coupling between the pinned and pinning layers is inversely proportional to the net pinning moment a reduced net magnetic moment increases the exchange coupling between the first AP pinned layer and the pinning layer. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. Since an AP pinned structure has more conductive material on the pinned side of the free layer than a single pinned layer the sense current field on the free layer is increased from the pinned layer side of the free layer as compared to a single layer simple pinned spin valve sensor and it is therefore still more difficult to properly bias the free layer. Further, more of the sense current is shunted instead of being conducted through the spacer layer.
The sense current field problem is still further exacerbated in both the simple pinned and AP pinned spin valves when the pinning layer is a metallic layer, such as platinum manganese (PtMn). Accordingly, there is a strong felt need to provide an AP pinned structure as well as a single pinned layer that shunts less sense current, improves the biasing of the free layer and shunts less of the sense current.
I have provided the pinned layer structure of a spin valve sensor with at least one layer of cobalt iron hafnium oxide (CoFeHfO) for improving the biasing of the free layer structure with reduced sense current shunting. In a simple pinned layer structure, including at least first and second pinned layers, the first pinned layer is the cobalt iron hafnium oxide (CoFeHfO) layer and the second pinned layer is a cobalt based layer which interfaces the spacer layer. In the AP pinned layer structure, which includes at least first and second AP pinned layers, the first AP pinned layer is the cobalt iron hafnium oxide (CoFeHfO) layer and the second AP pinned layer is cobalt based and interfaces the spacer layer. Since the first pinned layer or the first AP pinned layer is cobalt based and interfaces the spacer layer, the magnetoresistance of the spin valve is optimized. In a preferred embodiment the cobalt based layer is cobalt iron (CoFe).
As discussed hereinabove it is desirable to decrease the field on the free layer structure due to the sense current flowing through the pinned layer structure so as to improve a counterbalancing of other magnetic fields acting on the free layer structure. In this manner a zero bias point can be achieved on the transfer curve of the read head for promoting read signal symmetry. This decrease of field is achieved by making the second pinned layer or the second AP pinned layer of cobalt iron hafnium oxide (CoFeHfO). Cobalt iron hafnium oxide (CoFeHfO) is magnetically soft and has a high moment but yet has a very high resistance, in the order of 400 xcexcohm/cm. Accordingly, as the sense current IS flows through the sensor, practically none of the sense current flows through the second pinned or second AP pinned layer, so that practically no contribution is made to the net sense current field on the free layer structure. Since there is minimal sense current shunting through the second layer the magnetoresistive coefficient dr/R(magnetoresistance) is improved.
Further, for a simple spin valve sensor the oxide content in the cobalt iron hafnium oxide (CoFeHfO) second layer enables the second layer to serve as a specular reflector of conduction electrons through the spin valve sensor which still further increases the magnetoresistance of the spin valve sensor. Without the specular reflection some of the conduction electrons are lost from the path through the sensor causing a loss in the magnetoresistance of the sensor. The thickness of the cobalt iron hafnium oxide (CoFeHfO) second layer can be easily designed for providing the desired demagnetization field from the pinned layer structure for completely counterbalancing other fields acting on the free layer structure. The equivalent magnetic thickness of cobalt iron hafnium oxide (CoFeHfO) is virtually the same as an equivalent thickness of a cobalt based material, such as cobalt (Co) or cobalt iron (CoFe). An equivalent thickness means the equivalent thickness to nickel iron (Ni80Fe20). An actual thickness of a cobalt or cobalt iron (Co90Fe10) layer has an equivalent magnetic thickness of about 1.7 times its actual thickness. In the preferred embodiment the first layer of cobalt iron hafnium oxide (CoFeHfO) will have a greater magnetic thickness than the second cobalt based layer in either the simple pinned layer structure or the AP pinned layer structure.
It is important that the range of the oxide content in the cobalt iron hafnium oxide (CoFeHfO) be 10% to 30%. When the oxide content falls below 10% the desired high resistance is not obtained and when the oxide content is above 30% the second layer loses its softness and high moment necessary for counterbalancing other fields on the free layer structure. Cobalt iron hafnium oxide (CoFeHfO) is highly thermally stable in that it maintains its structure, even at 400xc2x0 C. This can be important since a read sensor may reach such a high temperature when it accidentally contacts an asperity on a rotating magnetic disk. The hafnium (Hf) and oxide contents of the cobalt iron hafnium oxide (CoFeHfO) give this material a nanocrystalline structure with mixed oxide and metallic phases which causes the very high resistance. As the grain size gets smaller the resistance increases. It is believed that the oxygen (O2) content oxidizes slightly with the iron (Fe) and cobalt (Co) content, but mostly combines with hafnium (Hf) to form hafnium oxide (HfO). Additional descriptive material on cobalt iron hafnium oxide (CoFeHfO) is found in Journal of Applied Physics, Vol. 81, No. 8, Part 2, dated Apr. 15, 1997 by Y. Hayagawa. A preferred cobalt iron hafnium oxide (CoFeHfO) is (Co90 Fe10)100-xHf5O10-30 where x equals 15 to 35. The invention applies to either top or bottom spin valves where a top spin valve has its pinned layer structure closer to the second read gap layer and the bottom spin valve has its pinned layer structure located closer to the first read gap layer.
An object of the present invention is to provide a composite pinned layer structure wherein one of the layers is a high resistance high magnetic moment layer for decreasing a sense current field from a pinned layer structure on a free layer structure.
Another object is to provide a pinned layer structure wherein magnetoresistance of a spin valve sensor is increased by having a cobalt based first layer interfacing a spacer layer and a second layer, which has a cobalt iron (CoFe) constituent, for still further increasing magnetoresistance, and hafnium (Hf) and oxide contents for providing a high resistance for minimizing sense current shunting and the oxide content for specular reflection of conduction electrons so as to further improve the magnetoresistance.
Still another object is to provide a method of making a spin valve sensor with the aforementioned pinned layer structure in a magnetic read head or a magnetic read and write head assembly.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.